Most people dread tax season and next year, there may be a whole new worry for nature lovers.

Conservation groups say the imperiled California sea otter may slip from the state's income tax form for voluntary contributions next year due to a drop in donations.

The sea otter contribution has been on the form for the previous three years and has raised nearly $750,000 for research projects. This year, California taxpayers must contribute at least $258,563 to the fund for it to remain on next year's income tax form, according to Monterey Bay Aquarium spokesman Ken Peterson. So far, donations are about 75 percent of last year's total of $245,000, he said.

The minimum $1 donation to the sea otter fund supports research into the decline of the sea otter, which is listed as threatened on the U.S. endangered species list.

In 2009, the average sea otter population numbered 2,813. The population must grow to 3,090 to be removed from the endangered species list. Last year, the California sea otter population last year dropped, in a three-year average, for the first time in more than a decade, Jim Curland of Defenders of Wildlife, said.

The decline may be caused by disease, pollution from farm and urban runoff, or a decline in the availability of their food sources, but more research is needed to confirm those theories.

The California sea otter is found mainly between San Mateo and Santa Barbara counties but has been seen as far north as the Oregon coast and as far south as Long Beach.